As is known, nuclear power plants of the pressurized-water type include steam generators in which the steam which drives the turbine-generator is produced from a secondary coolant by transfer of heat in a heat exchanger from a closed primary coolant system. A steam generator of this type consists of a bundle of vertically-oriented tubes inside a cylindrical shell. In certain steam generators, the tubes are inverted U-tubes fastened to a lower tube sheet. In other types of steam generators, the tubes are straight and are fastened to lower and upper tube sheets. In either case, the primary coolant from the reactor flows inside the vertically-oriented tubes and steam is generated in the secondary coolant outside the tubes and flows upwardly along the tubes within the cylindrical shell of the steam generator.
The tube bundle is ordinarily many feet high; and, consequently, the tubes must be supported laterally at several elevations along their length in order to limit flow-induced vibrations during normal operation and to limit stresses in the tubes during severe abnormal conditions such as earthquakes or tube ruptures.
The lateral support means for tubes in existing steam generator designs consists of a plurality of spaced tube support plates or egg-crate support grids. In the case of tube support plates, drilled or broached holes are provided through which the tubes pass with small diametral clearances. In order to limit fretting at the supports during operation, the radial clearances between the tubes and the lateral support device must be small and the tube support plates or grids are made of a different material than the tubes. Materials commonly used for lateral supports are carbon steel and ferritic stainless steel. The tubes are commonly a high nickel alloy.
Carbon steel has been found to be susceptible to corrosion in the secondary-side water chemistries of power plants. A result has been the phenomenon of "tube denting", in which build-up of the corrosion product (which has larger volume than the base metal) squeezes the tube at the support location and creates tensile stresses in the tube support plate or grid. Ferritic stainless steel is viewed as a means of overcoming tube denting but, based on limited experimental evidence to date, it is also apparently susceptible to corrosion in typical secondary-side water chemistries.
Tube support plates or grids are expensive to construct and, in part because of the small clearances at the supports, cause installation of tubes to be somewhat difficult, and consequently expensive.